1. Field of the Present Disclosure
The present disclosure relates to a surface treatment method for a metal material in which the surface of the metal material is roughened to make a resin component or the like reliably adhere to the metal material, and a force sensor obtained using the method.
2. Description of the Related Art
Sandblasting, which is a technique to roughen a metal surface by blowing fine sand grains to the surface, is known as a technique of the related art of forming a roughened surface on the surface of a metal component to improve adhesiveness between the metal component and a resin component.
When sandblasting is performed, first, a masking tape is attached to a metal surface and a desired portion of the metal surface is activated (roughened) by sandblasting. Next, abrasives used in the sandblasting and metal filings are removed and the masking tape is peeled off. Lastly, the entirety including the metal surface is washed. At this time, some of the abrasives and metal filings remain on the metal surface and when a resin component or the like adheres to a metal component, some of the abrasives and metal filings are regarded as foreign substances on an adhesive layer, which cause deterioration in yield rate in the process or inhibit the adhesion and weaken adhesion strength. In the worst case, a problem that the both components are peeled off from each other may easily arise.
On the other hand, as a surface treatment method for a metal material not using such sandblasting, for example, techniques of using a laser are known. An example of such techniques is disclosed in JP-A-2013-111881 and in JP-B2-4020957.
According to the techniques of using a laser, only a position to be activated may be targeted since the laser irradiation has high accuracy, and thus when sandblasting is replaced with laser processing, a masking tape is not required. Thus, a masking process and a washing process for a metal surface subjected to sandblasting can be omitted. In addition, since the washing process is not required, a defective mode such as deterioration in adhesion force between a resin component and a metal component caused by some of abrasives and metal filings from sandblasting does not occur.
In the technique disclosed in JP-A-2013-111881, laser irradiations are successively performed along a line, and the interval between the formed laser irradiation traces is 250 μm or less. This technique improves adhesiveness between a metal component and a resin component by forming a roughened surface on the surface of the metal component by a laser. Specifically, this technique adjusts the interval (hatching width) between adjacent roughened surfaces to 250 μm or less and the depth of unevenness that forms the roughened surface to 50 μm or less. A more preferable range of the hatching width is 100 μm or more and 250 μm or less as described in paragraph [0025] of JP-A-2013-111881. In addition, in the technique disclosed in JP-A-2013-111881, the laser beam irradiation range is preferably a circle diameter of 200 μm or less and most preferably 60 μm to 130 μm as described in paragraph [0030] of the JP-A-2013-111881. That is, in this technique, the each of laser irradiation ranges is set to be considerably wide.
In general, the formation of the roughened surface portion having a wide laser beam irradiation range and a wide hatching width on the surface of the metal member is applied in a case of making a resin member adhere to the roughened surface portion of the metal member using an injection molding technique of injecting a resin into a mold at a high pressure. Accordingly, when the resin member is attached to the metal member with an adhesive layer of an adhesive or the like interposed between instead of using the injection molding technique, a wide base metal region not processed by the laser beam remains on the surface of the metal member. Thus, the adhesive layer may be peeled off from this unprocessed region and sufficient adhesion strength may not be maintained.
Further, the publication JP-B2-4020957 discloses a technique of joining a metal member and a different member using a laser processing technique and in the metal member having a joined portion with the different member, the joined portion is formed in such a manner that the metal member is subjected to laser scanning processing in a predetermined scanning direction and then subjected to laser scanning processing in another scanning direction intersecting the scanning direction.
It is assumed that the technique disclosed in JP-B2-4020957 is used in injection molding. Based on such an assumption, when the metal surface roughened by a laser is intentionally elevated and a bridge between adjacent portions is formed, a resin enters under the bridge due to the pressure during the injection molding and is set, and thus an anchor effect is obtained. However, when the resin component is attached to the metal component with the adhesive layer of an adhesive or the like interposed between, the pressure during the injection molding cannot be used and as shown in FIG. 8, the adhesive or the like does not enter under the bridge and thus an air layer is formed under or near the bridge, where it should exhibit the conventional anchor effect. As a result, even when the resin member is attached to the metal member with the adhesive layer of an adhesive or the like interposed between, the area in which the adhesive layer is in close contact with the surface of the metal member is considerably reduced. Thus, sufficient adhesion strength may not be maintained and the resin member may easily be peeled off from the metal member.